External monitor not detected on Ubuntu 18.04

Solution 1:

I solved it by purging, then reinstalling the nvidia drivers

sudo apt-get purge 'nvidia*'
sudo add-apt-repository ppa:graphics-drivers
sudo apt-get update

Then using the Software & Updates tool's Additional Drivers tab and installing the latest nvidia drivers

Solution 2:

I solved the problem installing lightdm and set it as the default display manager.

In the process, something changes because is starting to detect external displays again even if you go back to GDM3.

You can follow the instructions here on how to make these changes:

• sudo dpkg-reconfigure gdm
• select the display manager (LightDM, MDM, KDM, Slim, GDM) you want to use by default and hit enter
• restart your computer

Solution 3:

Solved for me by updating to recommended Nvidea card drivers how-to.
To install all recommended Drivers:
sudo ubuntu-drivers autoinstall

Solution 4:

Sweet Merciful Crap! There is a solution without installing lightdm!

Update:

It's apparently a better solution to run X11 as root. The one-line fix is to add this to this to /etc/X11/Xwrapper.config:

needs_root_rights = yes

It's a one line fix in /lib/systemd/system/gdm3.service

Replace

[Service]

ExecStartPre=/usr/share/gdm/generate-config

with

[Service]

ExecStartPre=/usr/bin/xinit /usr/share/gdm/generate-config

Found Here:

https://bugs.launchpad.net/ubuntu/+source/gdm3/+bug/1716857

All my external displays are now detected and working on gdm3

My System:

Ubuntu 18.04.3 LTS

Lenovo T480

Intel® Core™ i7-8650U CPU @ 1.90GHz × 8

GeForce GTX 1070 Ti/PCIe/SSE2 via Razer Core X over Thunderbolt 3

GNOME 3.28.2